What is diskarbitrationd?

diskarbitrationd runs every time you plug in or eject a disc.

What is diskarbitrationd?

diskarbitrationd is the Disc Arbitration daemon. It manages mounting, unmounting, and ejecting discs and volumes on your Mac. Every time you plug in a USB drive, insert an SD card, mount a disc image, or eject a volume, diskarbitrationd handles it.

What does it do?

The daemon handles:

• Mounting: automatically mounting volumes when a disc is connected
• Unmounting: safely unmounting volumes before ejection
• Ejection: checking with apps to make sure no files are in use before a disc is removed
• Arbitration: deciding which file system handler should mount a volume (APFS, HFS+, FAT, NTFS, and others)
• Notifications: telling Finder and other apps when discs appear or disappear

Why does "disk not ejected properly" happen?

When you unplug a drive without ejecting it first, diskarbitrationd did not get a chance to flush pending writes and tell apps. This is why macOS warns you. Data could be lost if files were being written at the time.

Does it handle disc images too?

Yes. When you open a .dmg file, diskarbitrationd mounts the disc image as a virtual volume. It works with diskimages-helper and hdiejectd for this.

Does it use many resources?

No. It is light and event-driven. It reacts to disc events and is otherwise idle.

Should you worry?

No. It is needed for disc management on macOS. Without it, you could not mount or eject any volumes.

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